The present invention relates to an electrostatic printing apparatus comprising an improved printing electrode head.
U.S. Pat. No. 3,898,674 to Koch discloses improvements to a non-impact printer which is disclosed in U.S. Pat. No. 3,550,153 to Haeberle. In this printer, a pulsed electric field is applied between a shaping and base electrode through a donor sheet and a closely adjacent recipient sheet to transfer electrically conductive printing material particles from the donor sheet to the recipient sheet. A shield electrode is disclosed by Koch which cancels the fringe components of the pulsed electric field emanating from points on the surface of the shaping electrode outside the printing face in order to reduce the size of the printed mark and increase the resolution of the system. An important aspect of this prior art printer is that a gap must be maintained between the donor sheet and the recipient sheet. The shaping electrodes may be arranged in, for example, a dot matrix array. However, Koch does not disclose any specific method or apparatus for interconnecting and energizing the shaping electrodes of a dot matrix array for printing.
The present invention constitutes an improvement to an electrostatic printer which eliminates the need for a donor sheet and thereby the cost of printing. In such a printer, a rotary cylinder is coated with a liquid or dry layer of a developing substance comprising charged particles of pigment or the like and a recording sheet, which may be an ordinary sheet of writing paper, is fed in contact with the cylinder. A printing head is positioned closely adjacent to the cylinder so that the recording sheet is disposed between the cylinder and printing head. The printing head comprises a plurality of shaping electrodes arranged in a dot matrix configuration. A voltage applied to selected shaping electrodes attracts adjacent particles to the recording sheet for printing, with the particles being later thermally or otherwise fixed to the recording sheet to provide a permanent record.
A problem which has heretofore remained unsolved in this type of system is that since the developing substance is maintained in contact with the recording sheet charged particles will adhere to the recording sheet even in non-printing areas. This creates a smeary appearance and severely limits the contrast which can be provided by the printer. Another problem which has remained unsolved is that of interconnecting the shaping electrodes for selective energization. The packing density of the electrodes in a high resolution printer may be as high as 4-10 per millimeter, and the wiring problem has been extreme.